The objective of the research contained in this application is to elucidate how the structural features of carotenoids determine their photochemical properties and biological functions in vivo. Particular attention will be given to the roles of carotenoids in light-harvesting and photo-protection, which are essential for the survival of photosynthetic organisms. The systems to be studied include carotenoids in vitro and in pigment/protein complexes, obtained from different species of algae and prokaryotic and purple photosynthetic bacteria. These systems will be used, because reaction centers, and antenna carotenoid-containing pigment-protein complexes prepared from them, can be purified, pigment-reconstituted, pigment-exchanged, crystallized, and genetically altered via site-directed mutagenesis. X-ray crystallography, nuclear magnetic resonance, and resonance Raman spectroscopy will be used as direct structural probes. Kinetic and quantum mechanical models of the photochemical behavior of carotenoids will be tested using steady state absorption, fluorescence, fluorescence excitation, electron paramagnetic resonance, two-photon, and ultrafast-transient absorption spectroscopy. These experiments will be augmented by theoretical quantum mechanical computations, to allow a quantitative correlation of structure with spectroscopic observables and biological function. The specific aims of this proposal are: (1) To examine the structures of carotenoids in photosynthetic pigment-protein complexes; (2) To explore the relationships between the carotenoid structures and their spectroscopic properties; and (3) To determine the relationships between the carotenoid structures and the mechanisms by which they function in vivo. Several preliminary studies have already been carried out by us in support of the methods we propose to employ.